Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting diodes/devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
One application for phosphorescent emissive molecules is a full color display. Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors. In particular, these standards call for saturated red, green, and blue pixels. Alternatively the OLED can be designed to emit white light. In conventional liquid crystal displays emission from a white backlight is filtered using absorption filters to produce red, green and blue emission. The same technique can also be used with OLEDs. The white OLED can be either a single EML device or a stack structure. Color may be measured using CIE coordinates, which are well known to the art.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.
As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form.
As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level.
More details on OLEDs, and the definitions described above, can be found in U.S. Pat. No. 7,279,704, which is incorporated herein by reference in its entirety.
OLEDs with stacked multiple emissive layers are known and have been reported to produce white light, particularly, for commercial and residential lighting applications. See, for example, U.S. Pub. No. 2006/0006792 to Strip, and U.S. Pat. No. 8,777,291, assigned to Universal Display Corporation. In the UDC '291 patent, calculations showed that the light emitted for a stacked white OLED is a function of both the wavelength and the source position of the individual light emitting layers. Results indicate that R, G, and B sub-elements, arranged in different orders, have different extraction efficiencies and thus yield different color temperature and color rending indices (CRI) with other parameters staying the same. The emitting layer order of B-G-R (with R adjacent to the ITO anode) is said to lead to an optimal color balance.
U.S. Pub. No. 2007/0035240 to Yang et al. describes a stacked white OLED with a blue emitting layer sandwiched by two symmetric red layers, See, Table 1 of Yang. Tang sought to stabilize color temperature of the light with variation in lighting intensity and seeks to solve this problem by positioning red light-emitting layer on both sides of a blue light-emitting layer. Yang identifies select WOLED with the following emitting layer profiles red-blue-red, red-green-red, red-green-blue-green-red, red-green-blue-red, red-blue-green-red, blue-red-green-red-blue, or blue-green-red-blue. Moreover, Yang finds that if the two outermost emitting layers provide the same or similar color light, the WOLED has greater color stability.